Unsteady Behavior of a Laminar Separation Bubble Subjected to Wing Structural Motion

A combined research approach including wind-tunnel experiments and high-fidelity numerical simulations was taken to investigate the fundamental flow physics of separation for wing sections undergoing temporal motions. Detailed investigations for both a static and plunging wing section have been carried out for a modified NACA 64₃ − 618 airfoil at a nominal zero angle of attack for a chord-based Reynolds number of Re = 200k. For the static characterization, Infrared Thermography (IT) was considered in the experiments to locate the Laminar Separation Bubble (LSB) that forms on the suction side of the airfoil. This approach was compared to static pressure measurements and Particle Image velocimetry (PIV). For the unsteady investigation, a plunging motion with a reduced frequency of k = 0.67 and an amplitude of ℎ = 6% based on chord length was imposed on the wing. For the static wing, DNS exhibits a mean separation bubble larger than the experiments, mainly due to an earlier onset of transition that is attributed to non-zero free-stream turbulence (FST) in the experiments. To replicate the effects of FST in the experiment, very low-amplitude random disturbances are introduced in the DNS. This accelerates transition, which in turn decreases the mean separated region, matching remarkably well with the experiments. For the plunging wing, both the experiment and DNS capture a similar hysteretic behaviour for the bubble size and location during the plunging cycle. No bubble bursting is observed at these conditions, thus having a small impact on the global lift coefficient of the wing. Similar to the static case, the results from the DNS with random disturbances show good agreement with the experiments.